Why is snapping to the Grid in Blender 2.8 not (always) reliable for precision for centimeters?
A few things to note about 'Snap to Increment'


In Object Mode, it snaps the mean position of the pivot points of selected object(s), in Edit Mode, the mean position of directly or implicitly selected vertices.
It snaps to the grid increments you can see. (If you're zoomed in, the increments may be subdivisions of the increments when zoomed out)
If Absolute Grid Alignment is switched off, you are constrained to move elements by units of the smallest increment you can see. If switched on, you are constrained to move elements to the nearest grid point. (In the first case, elements won't necessarily wind up on a grid point)
Whatever your view, the snap is to an orthogonal projection down onto the grid you can see. If you're viewing at an angle, or in perspective, the snapped element won't necessarily appear to coincide with the grid you can see, until you adopt an orthogonal view straight down onto it.
If using in an orthogonal view, it will also snap in the dimension you can't see. (So for example, you're looking straight down Z, snapping in XY, starting off-grid in Z, there may be some unwanted movement to the nearest Z grid-point, unless you constrain the movement. (GShiftZ)


A few of these things might give you the impression the increment snapping 'isn't reliable', and can make it counter-intuitive to begin with . But it works as expected. 

EDIT: This answer doesn't really answer the question, since it has become one about precision.. but I'll leave it up..